Cfd Heat Exchangers

7/29/2019 Cfd Heat Exchangers

1/7

Simulation and CFD Analysis of heat pipe heat exchanger using Fluent to

increase of the thermal efficiency

M. H. SABER, H. MAZAHER ASHTIANICFD department

Fanavar Petro Arya engineering Co.

Tehran, Saharak Gharb

IRAN

[email protected] http://www.petroarya.com

Abstract: -In this paper, the heat pipe heat exchanger (HPHE) is considered and computational fluid dynamics (CFD) isused to analyze its evaporators performance and based on it, will be try to increase the thermal efficiency and to

optimize the distribution of fluid flow in this type of heat exchangers. The CFD principles which are made use in this

article are effective and appropriate methods that using finite volumes to solve the processes that consist of transportphenomena. The necessary numerical computations are accomplished by Fluent (the CFD solver program) and the

results are given in graphical representation. This analysis is done for an existing HPHE with the specified conditions of

inlet flow to evaporator and finally the comparison and conclusion are presented.

Key-words: -heat pipe heat exchanger, HPHE, simulation, CFD, Fluent.

1 IntroductionHeat pipes are two-phase heat transfer devices with high

effective thermal conductivity. Due to the high heat

transport capacity, heat exchanger with heat pipes hasbecome much smaller than traditional heat exchangers

in handling high heat fluxes. With the working fluid in a

heat pipe, heat can be absorbed on the evaporator region

and transported to the condenser region where the

vapour condenses releasing the heat to the cooling

media [1].

A heat pipe is an evaporation-condensation device

for transferring heat in which the latent heat of

vaporization is exploited to transport heat over long

distances with a corresponding small temperature

difference. The heat transport is realized by means of

evaporating a liquid in the heat inlet region (called theevaporator) and subsequently condensing the vapour in

a heat rejection region (called the condenser). Closed

circulation of the working fluid is maintained by

capillary action and /or bulk forces. The heat pipe was

originally invented by Gaugler of the General MotorsCorporation in 1942, but it was not, however, until its

independent invention by Grover [3, 4] in the early

1960s that the remarkable properties of the heat pipe

became appreciated and serious development work took

place. An advantage of a heat pipe over other

conventional methods to transfer heat such an a finned

heat sink, is that a heat pipe can have an extremely high

thermal conductance in steady state operation. Hence, a

heat pipe can transfer a high amount of heat over a

relatively long length with a comparatively small

temperature differential [1,2].

The increasing demand for energy efficiency in

domestic appliances (such as a dishwasher, air

conditioner, durable drier or fridge/freezer) and

industrial systems and devices is the main drive for

continuously introducing and/or improving heat

recovery systems in these appliances, systems and

devices. Heat transfer efficiency in such systems is the

primary factor for efficient performance of the whole

systems [5].

However, the design of the heat recovery systems

with heat pipe units is the key to providing a heat

exchanger system to work as efficient as expected.Without correct design of such systems, heat pipes are

not able to transport enough heat and may function as

an extremely poor thermal conductor in the systems [6].

Computational fluid dynamics is a powerful tool for

fluid dynamics and thermal design in industrial

applications, as well as in academic research activities.

Based on the current capabilities of the main CFDpackages suitable for industry (such as FLOTHERM,

ICEPAK, FLUENT and CFX) and the nature of

industrial applications, understanding the physics of the

CONTINUUM MECHANICS, FLUIDS, HEAT

ISSN: 1790-5095 183 ISBN: 978-960-474-158-8


2/7

processes, introducing adequate simplifications and

establishing an appropriate model are essential factors

for obtaining reasonable results and correct thermal

design [6].Having any knowledge about flow type, temperature

profile and turbulence zones in heat pipe heat exchanger

are very useful aids for reliable, high efficiency and

economical design or optimization. Although the most

existent designs use experimental methods, but as a

outcome of growth and development of numerical

methods and the softwares which can solve the PDE,

the tendency for analyses of fluid flow and heat transfer

are appeared. So because of expense and time wasting

of empirical assessments and hardness of turbulence

conditions verification and details in heat exchangers, a

reliable modeling is desired for HPHE designing.Computational fluid dynamics (CFD) is one of the

robust methods for fluid flow simulation that analyzes

the systems consists of momentum transfer, heat

transfer and chemical reactions (mass transfer). In fact,

CFD is more than a simulator because it reinforces

designers having good sights about the operations and

processes in the system. In this paper, goal is the

analysis of fluid conditions on the tubes bundle in the

HPHE using CFD packages and well make decisionhow flow enters to optimize distribution based on its

results.

2 Structure and operation of heat PipesThe main regions of the standard heat pipe are shown in

Fig. 1. In the longitudinal direction (see Fig. 1a), the

heat pipe is made up of an evaporator section and a

condenser section. Should external geometrical

requirements make this necessary, a further, adiabatic,

section can be included to separate the evaporator and

condenser.

The cross-section of the heat pipe, Fig. 1b, consists

of the container wall, the wick structure and the vapour

space.

Fig. 1 The main regions of the heat pipe

The heat pipe is characterized by the following:

(i) Very high effective thermal conductance.

(ii) The ability to act as a thermal flux transformer.

(iii) An isothermal surface of low thermalimpedance. The condenser surface of a heat pipe will

tend to operate at uniform temperature. If a local heat

load is applied, more vapour will condense at this point,

tending to maintain the temperature at the original level.

The overall thermal resistance of a heat pipe, defined

by equation (1), should be low, providing that it

functions correctly.

The operating limits for a wicked heat pipe, are

illustrated in Fig. 2.

(1)

Fig. 2 Limitations to heat transport in a heat pipe.

Each of these limits may be considered in isolation.

In order for the heat pipe to operate the maximum

capillary pumping pressure, Pc, max must be greater

than the total pressure drop in the pipe. This pressure

drop is made up of three components.

(i) The pressure drop Pl required to return the

liquid from the condenser to the evaporator.

(ii) The pressure drop Pv necessary to cause the

vapour to flow from the evaporator to the condenser.

(iii) The pressure due to the gravitational head, Pg

which may be zero, positive or negative, depending on

the inclination of the heat pipe.

For correct operation; P, P P P . Ifthis condition is not met, the wick will dry out in the

evaporator region and the heat pipe will not operate [7].

3 CFD modelingCFD methods consist of numerical solutions of mass,

Momentum and energy conservation with other

equations like species transport. Two main stages

comprise the solution of CFD problems. First, whole of


ISSN: 1790-5095 184 ISBN: 978-960-474-158-8


3/7

fluid field divides to small elements that their names are

mesh, and then partial differential equations that explain

transport phenomena in fluid flow are used for these

elements. Consequently, many non-linear equationsappear which have to solve simultaneously. Thesolution of these equations accomplish with numerical

algorithm and methods.Conservation equations for compressible flow are

[8]:

Continuity equation: 0 (2)

Reynolds equations:

..

..

..

(3)It is needed a model of the turbulence to solve above

equations. There are many equations to model

turbulence. k- model is one of the most reliable

models that exist. This model applies blew turbulence

parameters:

,

.

.

2. .

(4)

Where: C , C1 , C2 , L are coefficients of turbulent

kinetic energy. , t , eff are laminar, turbulence and

effective viscosity. Eij

is element deformation rate, is

turbulent dissipation rate, and k is turbulent kinetic

energy.

4 Heat pipe heat exchanger CFD

modelingIt was made a specific geometry for the HPHE in

dimensions of 150*180*150 (cm*cm*cm) that is based

on the basic design and with attention to needs and

process constraints in the operation unit of gas field of

South Pars in IRAN. After making geometry of HPHE

in Gambit (the CAD program), meshing of HPHEs

evaporator drew a volume which has relatively 1500000

cells. Then, this model entered in Fluent (CFD solver

program) to solve. In fluent user interface 3D doubleprecision (3dd), segregated solution method and k- turbulence model are used to simulate this CFD

problem.The schematic of HPHE geometry for this case study

is shown Fig. 2.

Fig. 2 case1: Model of HPHE (basic design)

There are 72 tubes in this heat exchanger that have been

placed in 6 rows with 12 tubes in any row. Inlet flow

conditions (physical specifications) which come frombasic design condition are given in table 1 and they are

same for all of cases.

Table 1physical specification of combustion gases for CFDmodeling

ValuePhysical parameters

793Temperature (K)

3.75Mass rate (kg/sec)

2*10-5

Viscosity (kg/m.s)

0.88Density (kgm/

3)

95Heat conduction (W/m.K)

So that is shown in Fig. 2, entrys dimensions of

HPHE is 30*30 (cm*cm) and flow enters in center of

tubes bundle. For simulation of evaporator where hot

fluid (combustion gases) accompanies pressure drop,

heat flux is supposed constant value. In Fig. 2 to 5 are

shown geometry of four different cases that this paper

purposes to compare with them. Case (1) is introduced

as basic condition and its geometry specifications

presented in Fig. 1. In the second case, dimensions of

cross area of entry have been double size. In the case (3)


ISSN: 1790-5095 185 ISBN: 978-960-474-158-8


4/7

with same dimensions to basic design, is used one

horizontal plate after entry. Finally, in 4th case one

imperfect cone is used in entry of HPHE to analyze the

distributed flow.In the assessment of this article, analysis of outlet

temperature of combustion gases as well as flow

distribution on the tubes bundle is presented.

Fig. 3 Case 2: increase of entry cross area

Fig. 4 Case 3: horizontal plate after entry

Fig. 5 Case 4: imperfect cone after entry

4.1 Distribution of flowWhen simulated model solved in Fluent and the residual

converged, the solutions are extracted and the results areillustrated by figures. Axial velocity distribution in

central plate of HPHE for basic design is shown in

Fig.6.

Fig. 6 Contours of axial velocity for case (1)

When heat flux is constant (-156568 W), temperature

value in HPHEs outlet will be 750K. This specifies that

combustion gases have temperature decrease about 43K.

Here for presentation and comparison, geometries andresults are given together and they are analyzed. In Fig.

6 to 9, axial velocity contours thorough of HPHE and in

central plate are illustrated. In Fig. 6 so that is

distinguished, from the begging to middle of HPHE, the

flow hasnt been distributed over the whole tubesbundle completely. Therefore, in the first look this

matter determines that it hasnt been used whole heat

exchangers volume effectively. In fact, some parts of

tubes bundles surfaces arent exposed to hot gas flow,

so the HPHE efficiency lowers. In Fig. 7 whosedimensions of entry cross area is double of basic design,

in comparison with case (1), the flow profile approaches

to fully distribution in shorter distance from entry. Set

of this materials, it could be say that the entry which has

larger cross area prepares better distribution on the

tubes bundle. Of course in industrial units there are

many constraints for cross area because growth of it

increases pressure drop for gas flow. Using the

distributers and baffles may leads to conduct the flow

on the tubes bundle. This paper considers two cases that

the baffles are used in those. One case uses a horizontal

plate after entry of HPHE which can divide flow to two

parts (case (3)), and in another case using one imperfect

cone conducts the flow in 3 paths comprises up, down


ISSN: 1790-5095 186 ISBN: 978-960-474-158-8


5/7

and middle of HPHE. Case (3) is better than case (1)

approaches to fully distribution. This means that case

(3) reaches to suitable distribution in shorter distance

from the beginning of HPHE in comparison with case(1), but according to the graphical depiction, case (2)

has still better distribution.




In the 4th case which has one imperfect cone baffle the

flow is divided to three parts. One path of flow is

conducted to the center of tubes bundle and other paths

go to up and down sections of HPHE symmetrically.

4.2 Temperatures distributionThe contours of temperature for all of cases are shown

in Fig. 10 to 13. Contours of temperature can explain

performance of HPHE clearly. Certainly temperatures

profiles are derived flows profiles. Using contours of

temperature that are exhibited graphically, the analysisof temperature differences in HPHE length is possible.

In Fig. 10 to 13 that are illustrated temperatures

profiles, the areas that arent affected by heat of

combustion gases are verified. According to Fig. 10

depend on case (1) it is considerable that the large tubes

area doesnt expose combustion gases and so heat

transfer is low and heat recovery is not appropriate.

About case (2), so that the Fig. 11 shows, temperatures

distribution is better and more developed so

temperatures profile approximates plug flow. In Fig. 12

and 13 which are concern two cases that are used

baffles, conditions of temperatures distribution aresuitable and acceptable, specially case (4) which has a

developed distribution and symmetrical profile oftemperature.


ISSN: 1790-5095 187 ISBN: 978-960-474-158-8


6/7

Fig. 10 Contours of temperature for case (1)




Finally, for a convenient comparison, table (2)

is given that reports average of outlet flowtemperature and temperature difference (T). Inlet

flow temperature is 793K for all of them.

Table 2 Outlet temperature and temperature difference for all

of cases

Tout T

Case 1 750 43

Case 2 747 46

Case 3 747.5 45.5Case 4 738 55

So that is realizable from table (2), case (4) with

imperfect cone baffle has highest rate of

temperature change that causes the highest heat

transfer and heat recovery between these cases.

Case (2) with double size entrys cross area is aftercase (4) in heat recovery capability. Therefore, it could

be state that type of the flows distribution is effective

parameter in HPHEs rate of heat transfer and

consequently in heat recovery quantity of it.

5 ConclusionsFinally here some considerations for performance

optimization in HPHE are presented:

1- Short baffles can be used in up and down sections

of HPHE to avoid bypass flows.

2- Entry cross area affects distribution of flow and

temperature. According to the results, although increase

of entry cross area helps to make better distribution, but


ISSN: 1790-5095 188 ISBN: 978-960-474-158-8


7/7

large scale of it may increase pressure drop and

operation costs.

3- Use of baffles is a very effective role for

appropriate development of flow and temperaturesprofiles. The results show that using an imperfect cone

with 1/5 diameter ratio, can optimizes performance of

HPHE very well.

4- A combined method thorough of these methods

can make highest efficiency. Use of the larger entry so

process constraints allow, as well as one imperfect cone

is a suitable way. By this method, the best distribution

of flow and temperature with minimum pressure drop

will be reached and so operation cost may decreases.

The above results and conclusions are usable in the

design and optimization fields directly and cause

increase of the thermal efficiency of heat pipe heatexchanger (HPHE) in both of the evaporator and

condenser.

References:

[1] Firouzfar E., Attaran M., "A review of heat pipe heatexchanger activity in Asia", Proceedings of world

academy of science, engineering and technology,

volume 30 July 2008 ISSN 1307-6884.

[2] Gaugler, R.S. US Patent 2350348. Appl. 21 Dec,

1942. Published 6 June 1944.[3] Grover, G.M. US Patent 3229759. Filed 1963.

[4] Grover, G.M. Cotter, T.P. and Erickson, G.F.

"Structures of very high thermal conductance", J. App.

Phys., Vol. 35, P. 1990, 1964.

[5] Sauciue I., Akbarzade A., Johnson P., Chracteristics

of two-phase closed thermosuphons for medium

temperature heat recovery applications, heat Recovery

Systems and CHP, P. 631-640, 15(7) 1995.

[6] Lin S., Broadbent J., McGlen R., "Numerical study

of heat pipe application in heat recovery sustems",

Applied Thermal Engineering V. 25(2005) 127-133.

[7] Reay D.A., Kew P.A., "Heat pipes Theory, Designand Application", Fifth Edition, Elsevier 2006.

[8] Versteeg, H. K., Malalasekera, W.,"An introduction

to computational fluid dynamics The finite volume

method", Longman Limited, England, 1995.


ISSN: 1790-5095 189 ISBN: 978-960-474-158-8

Cfd Heat Exchangers

Documents

Transcript of Cfd Heat Exchangers